1. Field
The present application generally relates to optical metrology of a structure formed on a semiconductor wafer, and, more particularly, to generating a simulated diffraction signal using a dispersion function relating process parameter to dispersion.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine one or more profile parameters of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after the fabrication of a structure on a semiconductor wafer, an optical metrology tool is used to determine the profile of the structure. By determination of the profile of the structure, the quality of the fabrication process utilized to form the structure can be evaluated.
In one conventional optical metrology process, a measured diffraction signal is compared to simulated diffraction signals. The simulated diffraction signals are generated using an optical metrology model. The optical metrology model includes a number of parameters that are varied in generating the simulated diffraction signals. Increasing the number of parameters of the optical metrology model can increase the accuracy of the optical metrology process when those parameters are not correlated to each other. However, parameters are typically correlated to each other to some degree. Thus, increasing the number of parameters of the optical metrology model that are varied can also increase the instability of the model.